Understanding why the expansion of the universe is presently
accelerating is one of the most important open questions in modern
cosmology. In this work we show that the presence of a temporal
electromagnetic field on cosmological scales generates an effective
cosmological constant which could be responsible for the acceleration.
Primordial electromagnetic quantum fluctuations produced during
electroweak scale inflation could naturally explain the presence of
this field and the measured value of the dark energy density. This
mechanism could be discriminated from a true cosmological constant by
observations of CMB and structure formation. In the same way as the
presence of matter or radiation in the Universe breaks global Lorentz
symmetry, the existence of dark energy could be signalling the
breakdown of electromagnetic gauge invariance on cosmological scales.